1. Field of the Invention
The present invention relates to small hollow borosilicate microspheres and processes for the production thereof.
2. Description of the Related Art
Generally, microspheres are very small spheres of material which are useful as fillers in the plastics industry. Microspheres may be made from siliceous material, ceramic, glass, plastic, or mineral. Specifically, microspheres may be made from borosilicate material. These microspheres may be solid or hollow.
The present application is concerned primarily with hollow siliceous microspheres, particularly those made from borosilicate material. Hollow borosilicate microspheres have wide application. They can be used in reflective paints and coatings. They can also be incorporated into molded plastic products, resulting in several advantages including cost reduction, controlled density of molded parts, improved workability of a finished product including nail and screw holding and sanding and finishing, and thermal insulation. The hollow borosilicate microspheres can also be used in such products as cultured marble for aesthetic purposes and to prevent cracking. These advantages of hollow microspheres are utilized in a number of plastic products, including simulated wood, auto body fillers, cultured marble, bowling ball cores, carpet backing, coatings, flotation devices, foams and elastomers, and spackling patching material.
Hollow glass microspheres are generally available with an average microsphere diameter of from 65 to 100 microns as measured by volumetric particle counters, although average microsphere diameters as low as 50 microns have been obtained. These hollow microspheres have a density around 0.2 g/cc to 0.3 g/cc, with larger hollow microspheres generally having a lower density than smaller hollow microspheres. For many uses, hollow microspheres of this size and density are quite well suited.
In the past, many different methods of producing hollow microspheres have been developed, as evidenced in publications in this area. For instance, U.S. Pat. No. 2,797,201 to Veatch, et al., discloses a process for forming hollow particles from film-forming plastic material including cellulose derivatives, thermoplastic synthetic resins, acrylic resins, and thermosetting resins. The film-forming material is dissolved in a suitable volatile solvent along with a latent gas material. The film-forming material is divided into fine droplets and dried at a temperature which induces the evolution of gas from the latent gas material. The expanding gas inflates the drying droplet to form hollow particles. This process produces small, lightweight particles; however the process is not used with a borosilicate material, and no method for making very small, lightweight borosilicate microspheres is shown.
U.S. Pat. No. 2,978,339 to Veatch, et al., discloses a process for forming hollow particles from finely divided, solid particles of material forming a glass upon fusion. The glass particles are mixed with a compound which liberates a gas at the temperature of fusion of the glass. The solid particles are introduced into the top of a furnace zone which has a stream of hot gas running up through it. The hot gas causes the particles to fuse and liberate gas, whereby the hollow particles are formed. The flow rate of the hot gas is adjusted so that the larger particles remain in the hot zone for a longer period of time than the smaller particles. While this reference shows the production of traditional-sized hollow glass particles, no reference to very small microspheres of 15 to 20 micron diameter is shown.
U.S. Pat. No. 3,365,315 to Beck, et al. purports to show a method for making hollow glass bubbles which vary from 5 to 300 microns in diameter and a method for making hollow glass bubbles which have an average true particle density between 0.05 to 1.2 g/cc. However, only bubbles having mean diameters between 41 microns and 42 microns and true densities between 0.42 g/cc and 0.57 g/cc were formed in the examples presented (mean diameters were calculated by averaging the range of diameters within which 90% of the bubbles fell). No bubbles having a small diameter and a low density are shown. The process includes heating glass beads fused in an oxidizing atmosphere to a temperature where the viscosity is between 10 and 10,000 poises for no more than 2 to 3 seconds. This is accomplished by dropping the particles through a heating zone. Again, small lightweight hollow particles are not shown, nor is a working method of making them.
U.S. Pat. No. 3,699,050 to Henderson discloses spray dried particulate feed material for the commercial production of hollow, spherical, unitary, discrete glass spheres. The precursor comprises hollow alkali metal borosilicate particles, substantially all of which have discontinuous skins, and a predominant number of which are attached to at least one other particle. The feed is then introduced to a spheridizing furnace to form the hollow spherical product. While this publication appears to show the production of relatively low density hollow material (about 0.37 g/cc true density), it does not show a small diameter particle having such a low density.
U.S. Pat. No. 4,119,422 to Rostoker discloses a gel method for producing cellular borosilicate bodies from an aqueous slurry of colloidal silica, caustic potash, boric acid and alumina. The slurry is dried, crushed, and calcined and quickly cooled, although the crushing (or milling) may be done after the calcination and cooling. Once crushed, the material is introduced into a cellulating furnace to form microspheres. This reference appears to specify neither the size nor the density of the product microspheres.
U.S. Pat. No. 5,069,702 to Block et al., discloses a method for making small hollow glass spheres. The method requires that a surfactant be added to the liquid glass precursor mixture or solution. The surfactant-containing solution is then formed into droplets and the droplets are heated to drive off water and generate gas to form hollow glass spheres, which are then cooled. Alternatively, the surfactant-containing solution can be spray dried, then heated to form glass spheres. Block reports obtaining microsphere diameters as low as 18 microns using this method. Block does not appear to report the density of those microspheres. We have used a very similar process to get microspheres of the same size. Those microspheres had a density of about 0.55 g/cc. So high a density is undesirable in some specific applications. Accordingly, Block does not appear to disclose a method of making small, low density, surfactant-free microspheres.
U.S. Pat. No. 4,778,502 to Garnier, et al., discloses a process for making hollow borosilicate glass microspheres. The microsphere diameters reported in Garnier range from 8 to 80 microns, with densities from 0.4 g/cc (for microspheres 8 to 35 microns in diameter (with 0.59 g/cc reported for microspheres having a mean diameter of 13 microns)) to 0.24 g/cc (for microspheres 30 to 80 microns in diameter). However, production of microspheres with small mean diameters and low densities (below 0.25 g/cc) is not disclosed.
One use for hollow microspheres is in making lightweight auto body putties and spackle. The microspheres add volume, while making the putty less dense and easier to shape when cured. The resultant product with hollow microspheres is lighter in weight. However, for auto body putties, spackle, and similar products, large microspheres of 65 to 75 microns mean diameter produce a relatively rough surface when compared to traditional putties containing talc or calcium carbonate. Talc and calcium carbonate are solid mineral materials and are therefore quite dense and hard, making these materials less desirable fillers than hollow borosilicate glass microspheres. It would be desirable to develop a filler material for putty which has a low density and fine grain. Such a filler would be used to produce a lightweight putty with a smooth finish. Small, low density, hollow borosilicate microspheres would be useful as such a filler.